The present invention relates to the preparation of 2,5-dialkyl-4-hydroxy-3(2H)-furanones, and especially, but not exclusively, to the preparation of 2,5-dimethyl-4-hydroxy-3(2H)-furanone, commercially known under the trade name FURANEOL.
FURANEOL is an important aromatic substance found in pineapples (J. O. Rodin et al, J. Food Science (1965) 30, 280). It has a strong, fruity fragrance and is suitable for use in flavouring various food products (U.S. Pat. No. 3,887,589 of the Mar. 6, 1975) on account of its excellent aromatic and organoleptic properties.
According to J. E. Hodge and colleagues (U.S. Pat. No. 2,936,308), FURANEOL can be obtained in small quantities from rhamnose and piperidine acetate; however, rhamnose is very expensive and hence this process is uneconomic. Processes for synthesizing FURANEOL from other starting products are also known. The majority of these are based on the preliminary formation of straight-chain aliphatic derivatives which are then modified to provide appropriate diketogroups, combined with a diol or two halogen atoms, in positions suitable to allow the subsequent cyclyzation into the furanone ring structure to form FURANEOL or its desired homologues (U.S. Pat. No. 3,629,292; Swiss Pat. No. 474,501). Difficulties with these processes, however, are that they involve many expensive operating stages and give rather low yields of the final products.
The object of the present invention is, therefore, to provide a convenient process for the preparation of 2,5-dimethyl-4-hydroxy-3(2H)-furanone and its 2,5-dialkyl homologues.
The present invention is based on the use of 2,5-dimethyl-dihydro-3(2H)-furanone: ##STR3## (C.A. 43, 1949, 5424i; C.A. 47, 1953, 7478c; C.A. 57, 1962, 16526d; C.A. 87, P 117769v; German Pat. No. 2,600,864) as a starting compound for the synthesis of FURANEOL. If the above starting compound could be oxidised, the corresponding diketo-derivative might be obtained and then converted to the enol form, i.e. FURANEOL, according to the general scheme; ##STR4## However, FURANEOL has characteristics typical of reducing agents, that is, it has a considerable tendency to be oxidised (B. Wilhalm et al, Chemistry & Industry (1965) 1629) and the choice of a suitable oxidizing agent for a direct reaction of the above type has been very difficult because of the difficulty in stopping the reaction at the desired stage and avoiding the formation of secondary products by further oxidation or degradation. A rather longer, but more controllable route, has therefore been developed.